Research Proposal; The best evidence that atherosclerosis is a proliferative lesion is the fact that plaques contain a large monoclonal population of cells. Although monoclonality can only arise through multiple generations of cell replication, there has been little progress in understanding how monoclonality arises or how cell growth is regulated at this critical time. Using a newly-developed PCR-based assay for X chromosome inactivation, I will first determine the cell type responsible for monoclonality in plaque. I will then study the X-inactivation patch architecture of normal vessels and a spectrum of evolving atherosclerotic plaques. This will determine whether the aorta is assembled in broad layered patches of X-inactivation before plaques form, or if monoclonal expansion instead occurs coincident with lesion formation. If monoclonal expansion is linked to plaque growth, this implies that the plaque smooth muscle cells are somehow unique compared to their polyclonal precursors. To determine if vascular injury, in general, selects a "proliferogenic" clone of smooth muscle cells, I will determine if multiple forms of hyperplastic vascular disease are monoclonal. Next, having identified the time course of monoclonal expansion, I will determine if increased expression of growth factors can account for this cell proliferation. Finally, I will begin to test the hypothesis that somatic mutations underlie atherosclerosis, by determining if plaques have evidence of genomic instability or mutations in tumor suppressor genes.